Devices for the protection of over tensioning in spring motors for window shades and other devices have been in use for many years. One common approach to this problem is found in various forms in U.S. Pat. Nos. 350,656 to Brooks, 823,452 to Vickery, 873,438 to John, and 982,444 to Smith. Each of these patents disclose some form of a nut or other internally threaded element which is movable along a threaded shaft in response to the rotation of the shaft. A spring is fixed to the shaft and is tensioned or unwound with the rotation thereof in a given direction. The nut is positioned on the shaft so that it will engage a stop at a predetermined point when the desired number of turns of the shaft is reached. This predetermined point is made to correspond to the maximum tension which can be imposed on the spring motor without damage. Once the designated number of rotations is reached, further rotation of the shaft is prevented by engagement of the nut or other movable member with the stop.
Another approach is found in U.S. Pat. No. 1,052,689 to Pitman. In this patent, an elongated shaft is provided having a threaded portion at one end and an attachment means at the other end. A spring, acting as the motor for the window shade, is secured to the attachment means of the elongated shaft. A drum having internal threads is disposed along the threaded portion of the shaft and movable axially therealong in response to rotation of the shaft. The other end of the spring is attached to the drum, and thus, movement of the drum in one direction will cause the spring to tension and in the other direction to unwind. Means are provided to engage the drum and prevents its axial movement after a desired number or rotations of the shafts so as to provide a fixed stop to prevent the spring motor from being tensioned beyond a predetermined amount.
One limitation of each of the devices described above is that they are rather bulky in configuration and may not be readily adapted to the streamlined tubes which house modern day window shade devices. In addition, each of these over tensioning means provides a fixed stop which restricts the shaft from any further rotation in the direction in which the spring is tensioned. It is contemplated that this could present a practical problem in the use of window shades embodying such devices, particularly by those not familiar with their operation. Once the point where the stop engages is reached, the user could inadvertently pull further on the shade and create severe damage to the entire window shade assembly.
It has therefore been an object of this invention to provide a clutch mechanism which prevents over tensioning of the spring motor of a window shade without creating a positive stop which restricts further rotation of the shaft on which the spring motor is disposed.
It is another object of this invention to provide a window shade motor having a clutch mechanism which is streamlined in configuration and may be easily inserted within the current designs of window shade assemblies.